Scanning probe microscopes can be used to measure electrical properties of the surface of a sample with high spatial resolution. For example, the local charge, contact potential difference, electrical potential, dielectric constant and polarizability are of interest. Not only high spatial resolution, but also high resolution of the measured quantity is desired.
Generally the term “Kelvin Probe Microscopy” (KPM) is used when the local contact potential is measured. The terms “Kelvin Probe Force Microscopy” (KPFM) or “Kelvin Force Probe Microscopy” are more specific since they indicate that the Kelvin principle works by detecting the force between the two oscillating capacitor electrodes.
T. Fukuma et al. in Rev. Sci. Instrum., Vol. 75, No. 11, pp. 4589-4594 describe a scanning probe microscope having a probe on a cantilever, an oscillator generating a master signal, and an actuator driven by the master signal and applying a mechanical force to the cantilever. The deflection of the cantilever is measured by a detector assembly generating an observed signal depending on the amplitude of the deflection. In addition to this, a DC voltage source and an AC voltage source are provided for applying a DC voltage and an AC voltage between the probe and the sample. In operation, the DC voltage is adjusted to compensate any electrostatic field between tip and sample. In this case the ac force at the same frequency as the ac voltage goes to zero. This allows determining electrical properties of the sample such as contact potential, electronic potential, or local charges.